The quantitative detection of minority species is often performed by measuring the charged species arising from the interaction of electrons with the molecules. The sensitivity of such techniques is directly proportional to the cross-section for the reaction involved. For example, in a mass-spectrometer the species M might be detected through the reaction: EQU e + M .fwdarw. M.sup.+ + Ze
With a cross-section of approximately 10.sup.-.sup.16 cm .sup.2 for a 70 eV electron. In contrast to this technique, some instruments make use of the electronegativity of the molecule to be detected, through a dissociative electron attachment process illustrated by a reaction such as: EQU e + AB .revreaction. (AB.sup.-).sup.* .fwdarw.A + B.sup.-
wherein AB is an electronegative molecule capable of disassociatively attaching electrons to form fragments A and B where both A and B may be atoms or molecules.
Reactions of this type have cross-sections ranging from approximately 10.sup.-.sup.13 cm .sup.2 to approximately 10.sup.-.sup.23 cm.sup.2, the general trend being for the cross-section to decrease with increasing energy of the attaching electron. Where attachment of very low energy electrons is possible, as is SF.sub.6 for example, the cross section is sufficiently large (approximately 3 .times. 10.sup.-.sup.14 cm.sup.2) for a species to be easily detected in a relatively simple electron drift tube device such as the electronegative gas detector described in U.S. Pat. No. 3,211,966, issued Oct. 12, 1965 and assigned to the assignee of the present invention. The sensitivity of such devices to other gases is generally much lower, however, due to the smaller attachment cross-sections.